Three different types of conventional forged gears for a transmission used in a motorcar, an agricultural machine, a construction machine or other industrial machine are illustrated in FIGS. 3(a) to 5(b). Each of the three types of the conventional forged gears is described below.
[1] First type of conventional forged gear:
FIGS. 3(a) and 3(b) show a first type of such conventional forged gear. This forged gear is produced by (i) forging a single gear material to form a gear body and a dog member and (ii) machining the gear body and the dog member to form spiral teeth for the former and dogs for the latter. As shown in FIG. 3(b), each dog of the dog member is so formed as to have a width which becomes greater in an upward direction (in FIGS. 3(a) and 3(b)), except for its upper portion which is tapered in the same direction. Also, there is a clearance A.sub.1 between the dogs and one surface of the gear body. This clearance is produced, when the gear material is forged, as a space which will make it possible for the cutting tool to completely cut the lower portions (as viewed in FIGS. 3(a) and 3(b)) of the grooves to provide the dogs.
[2] Second type of conventional forged gear:
FIGS. 4(a) and 4(b) show a second type of the foregoing conventional forged gear. This forged gear is produced by (i) forging two separate gear materials into a gear body and a dog member, respectively, (ii) machining the gear body and the dog member to form spiral teeth for the former and dogs for the latter, and (iii) combining or joining the gear body and the dog member together by press fitting, welding or other suitable method.
As is the case with the first-type conventional gear of FIGS. 3(a) and 3(b), each dog of the dog member of the second-type conventional gear of FIGS. 4(a) and 4(b) is so formed as to have a width which becomes greater in an upward direction (in FIGS. 4(a) and 4(b)), except for its upper portion which is tapered in the same direction.
The gear body and the dog member combined together are subjected to heat treatment. The heat treatment may result in deforming the final product (gear). Thus, to prevent a deformation of it, the gear body and the dog member are produced such that the surfaces of the two entities to be brought into contact with each other (by combining the two entities together) will have relatively small areas. Also, there is a clearance A.sub.2 between the dogs and one surface of the gear body. This clearance is produced, during the forging of the gear material for the dog member, as a space which will not allow downwardly-projecting burrs resulting from the cutting of the grooves to provide the dogs to touch the gear body or will allow such burrs to touch the gear body and bend thereon, when the gear body and the dog member are combined together. That is, the clearance A.sub.2 makes it possible closely to combine the two entities together.
[3] Third type of conventional forged gear:
FIGS. 5(a) and 5(b) show a third type of the foregoing conventional forged gear. This forged gear is produced by (i) forging a single gear material to form a gear body and a dog member with half-finished dogs each having a particularly rough-finished lower portion (as viewed in FIGS. 5(a) and 5(b)), (ii) coining the whole in a cold process, and (iii) machining the gear body to form spiral teeth therefor, as well as machining the dog member to completely finish its dogs.
As illustrated, each dog of the dog member is so formed as to have a uniform width except for its tapered upper portion. Also, there is a clearance A.sub.3 between the dogs and one surface of the gear body. This clearance is produced, during the forging of the gear material, as a space which will make it possible for the cutting tool to completely cut the lower portions (as viewed in FIGS. 5(a) and 5(b)) of the grooves to provide the dogs, when completely finishing the lower portions of the dogs.
Drawbacks of the first type of conventional forged gear:
However, the fact that the dogs of the dog member of the above-mentioned first type of conventional forged gear are formed by using a cutting tool leads to the following drawbacks of the gear:
A. It is possible that the top of the dog, or the top of the tapered upper portion of the dog, may not lie in the exact middle of the width of the entire dog. PA1 B. It is possible that the top of the dog may be formed more or less flat. PA1 C. It is possible that the dog may have appreciable burrs, particularly on the surface of its tapered upper portion, which is formed by chamfering. PA1 D. As previously mentioned, the clearance A.sub.1 is produced, when the gear material is forged, as a space which will make it possible for the cutting tool to completely cut the lower portions of the grooves to provide the dogs. However, since this clearance results in making longer the axial dimension of the entire gear, the clearance makes it impossible or very difficult to produce a compact and lightweight transmission. To be concrete, if the gear is an ordinary one used for a manual transmission for a motorcar, the clearance A.sub.1 is usually set at approximately 3 millimeters. Thus, if the manual transmission is a five-speed transmission, the use of five gears of the first type each having the clearance A.sub.1 will result in an approximate total clearance of as great as 15 millimeters and, hence, a compact manual transmission may not be produced. Also, if an impact is given to the gear in an axial direction thereof (i.e., in a vertical direction in FIGS. 3(a) and 3(b)) due to, for example, an undesirable or unsuitable gearchange, the presence of the clearance A.sub.1 may cause the gear to be considerably damaged. PA1 E. If the gear body and the dog member are combined or joined together, by press fitting the dog member around the boss portion of the gear body (i.e., the portion thereof projecting vertically in FIG. 4(a), or in the axial direction of the gear), such that the inner surface of the dog member and the outer surface of the boss portion make a relatively large contact area, then the dog member may be liable to break. In contrast with this, if the press fitting of the dog member around the boss portion of the gear body is so made that the inner surface of the dog member and the outer surface of the boss portion make only a relatively small contact area, then it is possible that the dog member may be prone to removal from the gear body during the operation of the gear. Thus, if the gear body and the dog member are to be joined together by press fitting, special care must be taken, before forging gear materials, in selecting the optimum sizes of a dog member and of a boss portion of a gear body which will lead to the optimum press fitting of the dog member around the boss portion. Also, in such a case, careful attention must be given in selecting the pressure to be applied to the dog member for the press fitting thereof. PA1 F. On the other hand, an attempt at welding the gear body and the dog member sometimes results in an incomplete joining of the two entities. Also, the two entities are subjected to thermal stress during welding, and the stress certainly results in changing the sizes of the two entities.
The above-mentioned three drawbacks combine to prevent, for example, the driver of a motorcar with a transmission using the gears from performing gear-change very smoothly and comfortably.
Drawbacks of the second type of conventional forged gear:
Also, the above-mentioned second type of conventional forged gear has the same drawbacks as the above-mentioned drawbacks A, B and C of the first type of conventional forged gear for the same reason as the first type of conventional gear. With regard to the above-mentioned drawback D, if the gear is an ordinary one used for a manual transmission for a motorcar, the clearance A.sub.2 is usually set at at least approximately 1 millimeter. Thus, if the manual transmission is a five-speed transmission, the use of five gears of the second type each having the clearance A.sub.2 will still result in a detectable amount of total clearance, namely, an approximate total clearance of at least 5 millimeters.
Additionally, if the gear body and the dog member are combined, or joined, together by press fitting or welding, the following drawback E or F will result:
Drawbacks of the third type of conventional forged gear:
Although in a lesser degree than the foregoing first and second types, the foregoing third type of conventional forged gear also has the same drawbacks as the foregoing drawbacks A, B and C of the first type of conventional forged gear for the reason that a cutting tool is used to completely finish the dogs. Also, the clearance A.sub.3 prevents a compact transmission from being produced. Moreover, since each dog of the dog member has a uniform width except for its tapered upper portion, it is possible that the dogs of the dog member may disengage from the dogs of a dog sleeve (not shown) when the gear is being used in a transmission.